I have 2003 Altima 2.5S with 129k miles on it The engine light came ON last day. Read code P0420 CAT efficiency below threshold.... I reset the code and the light gone. I drove the car 10 days and then light came back again with the same code I reset it again then after couple of trips it came back again...
Any suggestion what causing it coming back again and again If CAT is bad or BAD O2 sensor then it should come back right away when the car will start. Why it take 8-10 days to come back again with the same code??? Any help will be appreciated

You need a new cat. convertor. The light will not come on until the computer test the system. This requires a specific drive pattern to complete. The only time the light will come on immediately on startup would be for a sensor that was disconnected or shorted out.

The scanner pointed out Bank1 sensor which is on the front. I have a friend who runs an auto-parts shop; he recommended to get the O2 sensor replaced first, as it is the case generally. I got this done about three months ago, the check engine light has been off ever since!